Add comprehensive GitHub Copilot instructions for text-cleanup repository

.github/copilot-instructions.md

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+# Text Cleanup Tool
+Text Cleanup Tool is a static HTML5, CSS, and JavaScript web application that cleans messy text from unwanted formatting, special characters, and hidden artifacts commonly found when copying from AI tools, PDFs, websites, and documents.
+
+Always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.
+
+## Working Effectively
+- Bootstrap and setup the repository:
+  - `cd /path/to/text-cleanup`
+  - `npm install` -- installs local http-server (takes ~2 seconds)
+- Run the development server:
+  - **Primary method**: `npm run dev` or `npx http-server -p 3000 -o`
+  - **Alternative method**: `python3 -m http.server 8000`
+  - **Manual method**: `npx http-server -p 3000` (without auto-opening browser)
+- Access the application:
+  - Primary server: http://localhost:3000
+  - Python server: http://localhost:8000
+- No build step required - this is a static site with no compilation needed.
+
+## Validation
+- **CRITICAL**: Always manually test the complete user workflow after making ANY changes to the application:
+  1. **Text Processing Test**: Paste text with formatting (markdown **bold**, *italic*, smart quotes "", em-dashes —) into the input area
+  2. Click "Clean Text" button and verify the output removes formatting and normalizes characters
+  3. **Copy Functionality Test**: Click "Copy to Clipboard" and verify the button shows "Copied!" feedback
+  4. **Clear Functionality Test**: Click "Clear All" and verify both text areas are cleared and focus returns to input
+- **Manual UI Testing**: Always test in a browser by navigating to http://localhost:3000 and exercising the above workflow
+- The application runs entirely client-side - no server processing is involved
+- No automated tests exist - manual validation is the only testing method
+
+## Common Tasks
+The following are outputs from frequently run commands. Reference them instead of viewing, searching, or running bash commands to save time.
+
+### Repository Root Structure
+```
+text-cleanup/
+├── index.html          # Main HTML file - contains UI structure
+├── script.js           # JavaScript functionality - TextCleanup class
+├── styles.css          # CSS styles - responsive design
+├── package.json        # Project configuration - contains dev scripts
+├── vercel.json         # Vercel deployment config
+├── README.md           # Documentation
+├── .gitignore          # Git ignore patterns
+└── yarn.lock           # Dependency lock file
+```
+
+### Key Files and Their Purpose
+- **index.html**: Main application interface with text input/output areas and control buttons
+- **script.js**: Contains TextCleanup class with methods:
+  - `cleanText()`: Main processing pipeline
+  - `removeHiddenCharacters()`: Strips invisible Unicode
+  - `normalizeWhitespace()`: Consolidates multiple spaces
+  - `removeFormatting()`: Removes markdown formatting
+  - `cleanSpecialCharacters()`: Normalizes quotes and dashes
+  - `copyToClipboard()`: Handles copy functionality with fallback
+- **styles.css**: Responsive CSS with modern design, button states, and mobile support
+
+### package.json Scripts
+```json
+{
+  "scripts": {
+    "dev": "http-server -p 3000 -o",
+    "dev:vercel": "vercel dev",
+    "build": "echo 'No build step required'",
+    "start": "echo 'Static site - no start command needed'"
+  }
+}
+```
+
+### Development Dependencies
+- **http-server**: Local development server (installed via `npm install`)
+- **No other dependencies**: Pure vanilla JavaScript application
+
+## Deployment
+- **Primary**: Vercel (configured via vercel.json)
+- **Alternative platforms**: Netlify, GitHub Pages, any static hosting
+- **Manual deployment**: Upload index.html, script.js, styles.css to any web server
+- No build process required - files are served directly
+
+## Code Architecture
+- **Frontend only**: No backend server or API calls
+- **Vanilla JavaScript**: No frameworks or libraries used
+- **ES6+ features**: Uses classes, arrow functions, async/await
+- **Browser compatibility**: Modern browsers with clipboard API support and fallback
+- **Processing pipeline**: Input → Hidden chars removal → Whitespace normalization → Formatting removal → Special character cleanup → Output
+
+## Making Changes
+- **Always test locally first**: Start dev server and manually validate changes
+- **Test all user scenarios**: Input text → Clean → Copy → Clear workflow
+- **No linting/formatting tools**: Follow existing code style manually
+- **No automated tests**: Manual browser testing is the only validation
+- **Static files only**: Changes to HTML/CSS/JS are immediately reflected after refresh
+
+## Common Debugging
+- **Server won't start**: Install http-server locally with `npm install`
+- **Copy functionality**: Modern browsers require HTTPS for clipboard API (uses fallback in HTTP)
+- **Text not cleaning**: Check JavaScript console for errors in the processing pipeline
+- **UI issues**: Inspect CSS styles - responsive design uses CSS Grid and Flexbox
+
+## Timing Expectations
+- **npm install**: ~2 seconds (installs only http-server)
+- **Server startup**: Immediate (~1 second)
+- **Application load**: Immediate (static files)
+- **Text processing**: Instantaneous (client-side JavaScript)
+- **No build times**: Static site requires no compilation
+
+## File Change Impact
+- **index.html changes**: Affects UI structure and layout
+- **script.js changes**: Affects text processing logic and user interactions  
+- **styles.css changes**: Affects visual appearance and responsive behavior
+- **package.json changes**: Affects development server and deployment configuration